The present invention relates to glass release coatings that are used in the manufacture of glass. In particular, it relates to glass release coatings comprising an oxide matrix and nitride particles after curing; parts that are used for manufacturing glass that are coated with the coating; and a method of applying the glass release coating to a part.
During glass manufacturing, glass is poured, cut into lengths, transferred to and from processing sites, shaped into preliminary forms, blown, molded and cooled. During this process, various parts of the manufacturing machinery are exposed to the hot, molten glass. These parts are made of materials that may adhere to the glass; undergo thermal degradation; or wear unacceptably causing various problems in the glass manufacturing process. For example, parts that adhere to glass introduce flaws that may serve as stress risers and lead to the fracture of the glass. Innocuous flaws are often objectionable because the clarity of the glass makes these flaws easily visible. In turn, the glass product must be rejected and discarded.
One approach to facilitating the handling of glass during manufacturing, is to make the machinery parts from expensive materials such as platinum and zirconium alloys. Other techniques involve swabbing the machinery parts with a swabbing agent or applying lubricants to the surfaces of those parts which come into contact with the glass. Swabbing is a manual operation which produces loose residue that is easily transferred to the glass. In turn, it becomes necessary to discard the glass product from several of the first cycles after the swabbing operation.
Typical swabbing agents are oily liquids containing a graphite lubricant dispersed in a predominantly hydrocarbon carrier. The carrier is burned or evaporated off leaving a loose lubricant coating on the swabbed surface. In order to minimize the manual labor involved in applying the lubricant, several attempts have been made to try to automate the swabbing process.
Others have tried to simplify the swabbing process by substituting powdery mold release agents. In one instance, boron nitride particles were used in a mold dressing. However these mold dressing and powdery mold release agents lacked a necessary binder to hold the agent in place. In turn, loose lubricant particles were quickly lost and thus frequent re-application became necessary. Moreover, the loose lubricant particles introduced flaws into the glass product and were the cause for rejection of the glass product when the particles adhered to the glass or became incorporated into it.
An alternative approach was to use organic binders coupled with a lubricant. This approach failed to provide a lasting lubricated surface that contacted the glass, nor was the need for swabbing eliminated. In addition, these organic binders undergo thermal decomposition as a result of the molten glass coming into contact with the binder, limiting their life time.
In order to provide good thermal resistance, inorganic binders were coupled with lubricants. These binders typically contain silicates or phosphates of alkali, alkaline, or other metal oxides which have a tendency to react with the hot or molten glass.
Various structures including coatings have been fabricated from nitride powders including mixtures of nitride and oxide powders (U.S. Pat. No. 4,904,626, U.S. Pat. No. 4,997,605 and U.S. Pat. No. 5,134,098). In fabricating these structures, a binder is added to nitride powders or mixtures of nitride powders and oxide powders to provide green strength (in the unfired condition), to promote sintering during firing, and to act as a cement, gluing the particles together. This binder may be an oxide-forming compound and after firing may represent up to 15% of the structure by weight. However, these structures are not suitable for forming permanent, strong, durable, mold-release coatings on machinery parts because they must be fired at temperatures that are extreme (greater than 1200xc2x0 C.) with respect to the parts. Such temperatures would melt, warp, or cause the parts to lose their tempering. In addition, once the coating is fully fired, the structures tend to be discontinuous having large pores which limits their strength, durability and glass-release ability.
Carborundum has a product line known as COMBAT Boron Nitride Coatings that function as mold release agents that employ an alumina binder. These coatings employ very coarse boron nitride particles (5-30 microns) which do not allow one to apply the coating at thicknesses less than about 25 micrometers. Typically, the coatings are applied at thicknesses of 75 micrometers to several centimeters thick. Once the coating is cured, it comprises 75% boron nitride and is soft and unable to resist mechanical wear. To have these types of coatings applied at these thicknesses to glass manufacturing parts would cause the machinery parts to bind and fail to operate. In addition, the application of these coatings requires careful processing involving many hours of drying and stages of firing to keep the coating from falling off of the part.
Therefore, it is an object of the present invention to provide coated parts for glass manufacturing having a coating that can withstand high temperatures, has excellent durability, and does not adhere to hot or molten glass.
Another object of the invention is to provide a part that is coated with a material having excellent bond strength and metal to metal lubricity.
Another object of the invention is to provide a coating that, when cured, provides a continuous oxide matrix that surrounds nitride particles.
Another object of the invention is to provide a coating comprising an oxide matrix and nitride particles that can be cured at temperatures less than about 1200xc2x0 C. yet be able to withstand the high temperatures of molten glass.
Another object of the present invention is to provide a part coated with a material that is non-toxic and will not carry over or contaminate the glass.
Another object of the invention is to provide a part that has a coating that is applied off of the manufacturing line.
Another object of the invention is to provide a coating that requires minutes to a couple of hours to apply.
The aforementioned and other objects were achieved by the present invention which is a thermally stable, glass release coating comprising from about 25% to about 99.8% by weight of an oxide matrix and from about 75% to about 0.2% by weight of nitride particles after curing. The oxide matrix is prepared from a solution comprising from about 10% to about 90% by weight of a carrier liquid; from about 0.1% to about 30% by weight of a liquid stabilizer; from about 4.5% to about 40% by weight of an oxide forming compound; from about 0% to about 4% by weight of an oxide stabilizer; and from about 0% to about 2% by weight of a corrosion inhibitor. The thermally stable, glass-release coating is applied to a part such that the part is coated with the coating and cured at a temperature less than about 1200xc2x0 C. The resulting part is useful for glass manufacturing because the coating exhibits excellent glass-release properties and reduces and/or eliminates the need for swabbing agents.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention will be obtained by means of instrumentalities in combinations particularly pointed out in the appended claims.